Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest.
Where replacement of a heart valve is indicated, the dysfunctional valve is typically cut out and replaced with either a mechanical valve, or a tissue valve. Tissue valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. The most common tissue valves are constructed with whole porcine (pig) valves, or with separate leaflets cut from bovine (cow) pericardium. Although so-called stentless valves, comprising a section of porcine aorta along with the valve, are available, the most widely used valves include some form of stent or synthetic leaflet support. Typically, a wireform having alternating arcuate cusps and upstanding commissures supports the leaflets within the valve, in combination with an annular stent and a sewing ring. The alternating cusps and commissures mimic the natural contour of leaflet attachment. Importantly, the wireform provides continuous support for each leaflet along the cusp region so as to better simulate the natural support structure.
A conventional heart valve replacement surgery involves accessing the heart in the patient's thoracic cavity through a longitudinal incision in the chest. For example, a median sternotomy requires cutting through the sternum and forcing the two opposing halves of the rib cage to be spread apart, allowing access to the thoracic cavity and heart within. The patient is then placed on cardiopulmonary bypass which involves stopping the heart to permit access to the internal chambers. Such open heart surgery is particularly invasive and involves a lengthy and difficult recovery period.
Some attempts have been made to enable less traumatic delivery and implantation of prosthetic heart valves. For instance, U.S. Pat. No. 4,056,854 to Boretos discloses a radially collapsible heart valve secured to a circular spring stent that can be compressed for delivery and expanded for securing in a valve position. Also, U.S. Pat. No. 4,994,077 to Dobbin describes a disk-shaped heart valve that is connected to a radially collapsible stent for minimally invasive implantation.
Recently, a great amount of research has been done to reduce the trauma and risk associated with conventional open heart valve replacement surgery. In particular, the field of minimally invasive surgery (MIS) has exploded since the early to mid-1990s, with devices now being available to enable valve replacements without opening the chest cavity. MIS heart valve replacement surgery still typically requires bypass, but the excision of the native valve and implantation of the prosthetic valve are accomplished via elongated tubes or cannulas, with the help of endoscopes and other such visualization techniques.
Some examples of more recent MIS heart valves are shown in U.S. Pat. No. 5,411,552 to Anderson, et al., U.S. Pat. No. 5,980,570 to Simpson, U.S. Pat. No. 5,984,959 to Robertson, et al., PCT Publication No. 00/047139 to Garrison, et al., and PCT Publication No. WO 99/334142 to Vesely. Although these and other such devices provide various ways for collapsing, delivering, and then expanding a “heart valve” per se, none of them disclose an optimum structure. For instance, the publication to Vesely shows a tissue leaflet structure of the prior art in FIG. 1, and an expandable inner frame of the invention having stent posts in FIGS. 3A-3C. The leaflets are “mounted to the stent posts 22 in a manner similar to that shown in FIG. 1.” Such general disclosures as in Vesely stop short of explaining how to construct a valve in a manner that maximizes long-term efficacy. In particular, the means of attaching the leaflets to the MIS stent is critical to ensure the integrity and durability of the valve once implanted. All of the prior art MIS valves are inadequate in this regard.
Another problem with MIS valves of the prior art is their relatively large radial dimension during implantation. That is, these valves all utilize one or more radially-expanding stents, and the assembly must be compressed radially and then passed through the lumen of a large bore catheter. Reducing the radial profile of the constricted valve via radial compression is problematic and conflicts with the need for sufficient circumferential length of the valve in its expanded state to fit within an adult heart valve annulus. Moreover, radial compression of the stent in combination with a biological valve must be done with great care so as not to damage the valve.
Tubular stents in general are known, typically constructed of a tubular metal lattice that has a normal, relaxed diameter and is compressed for insertion into a vein or artery. Upon expulsion from the end of a catheter, the tubular metal lattice expands to its original larger diameter in contact with the vessel wall. It is important to note that there is no regulation of the self-expansion of the stent. Furthermore, stents of the prior art expand outward to the size of the expansion balloon, or until meeting resistance from the surrounding vessel, and so their final size varies. Minimally-invasive heart valves that use conventional tubular stents that are self- or plastically expanded outward by a balloon do not have a positive maximum size limit; that is, there is no structure preventing further expansion of the stent. The final diameter of the valve is determined by the maximum size of the balloon, which may over-expand into the surrounding annulus leading to a less than optimum fit. Alternatively, the surrounding annulus may provide sufficient resistance to further outward expansion of the balloon so as to stop expansion of the heart valve prior to its optimum size. That is, the valve may exhibit a loose fit in the annulus, potentially leading to migration.
Some MIS valves of the prior art are intended to be used without removing the natural valve leaflets. Sometimes the natural leaflets are heavily calcified, and their removal entails some risk of plaque particles being released into the bloodstream. Therefore, some of the MIS valves are designed to expand outward within the annulus and native leaflets, and compress the leaflets against the annulus. In doing so, a relatively uneven surface against which the valve is expanded outward is created. This irregularity creates sizing problems, and also may adversely affect the circularity of the expanded valve which negatively affects the valve efficacy by impairing leaflet coaptation.
Despite some advances in MIS valve design, there remains a need for a valve that can be constricted into a smaller package without damaging the biological valve within, and which can be reliably expanded generally into a tube against the relatively uneven surface of the annulus or annulus and intact native leaflets.